U4080B TEMIC Semiconductors, U4080B Datasheet - Page 6

• Manufacturer Part Number: U4080B
• Description: Voice-Switched Circuit for Handsfree Operation
• Manufacturer: TEMIC Semiconductors

Transmit and Receive Attenuators
The transmit and receive attenuators are supplementary
in function. This means that when one is at maximum
gain, the other is at maximum attenuation, and vice versa.
That is, both are never on or off. They are controlled by
the voltage of ACF (attenuator control filter) at Pin 25
being supplied by attenuator control as shown in figure 1
The ACF voltage is provided by the attenuator control
block, which receives the three inputs given below:
D
D
D
The response of the attenuators is based on the difference
between the ACF voltage and V
(
the receive attenuator is fully off (T mode). If
150 mV, the circuit is in the R mode. If
the circuit is in the idle mode, and the two attenuators are
at gain settings approximately half way (in dB) between
their fully on and fully off positions.
Three resistors R
gain and attenuation values. R
according to its value relative to RTG and RG. Figure 4
U4080B
6 (15)
D
94 7864 e
V
R-T comparator
Transmit detector comparator
Volume control
ACF
Transmit
Transmit
–20
–40
Receive
Receive
) is
20
Comp
0
T-R
[
0
6 mV, the transmit attenuator is fully on and
D
1
VR
, RTG and RD, determine maximum
ACF
Figure 2.
= 150 mV
RG ( k
100
Transmit
Transmit
Transmit
1
Comp
W )
effects both attenuators
Idle
Idle
Det
D
VR
CC
Preliminary Information
ACF
. If the difference
D
V
= 8 mV
ACF
D
[
1000
V
ACF
75 mV,
Affects
Affects
No Effect
No Effect
[
Volume
Control
D
D
shows the variations versus the ratio of other resistors to
R1. RTG affects the gain and attenuation of only the
transmit attenuator according to the curves of figure 2,
while RG affects only the receive attenuator according to
figure 3 Gain difference from on to off, according to the
figures, is a reasonably constant 45 dB until the upper
gain limit is approached. A value of R1 = 30 k is recom-
mended as a starting point, and then the values of RTG
and RG selected to suit the particular design goals.
The input impedance of the attenuators (at TI and RECI)
is typically 5.0 k , and the maximum input signal which
will not cause output distortion is 250 mV
The 4 k
figure 1) filters out high-frequency components in the
receive path. This helps to minimize high-frequency
acoustic feedback problems which may occur if the filter
were not present.
The filter’s insertion loss is 1.5 dB at 1.0 kHz. The outputs
of the attenuators are inverted from their inputs. Referring
to the attenuator control block, the
output is determined by three inputs. The relationship of
the inputs and outputs is summarized in the following
truth table:
V
V
ACF
ACF
94 7863 e
20
20
40
0
resistor and 0.01 F capacitor at RECO (in
0
D
50 – 150 mV
50 – 150 mV
VT
75.0 mV
6.0 mV
D
D
ACF
V
V
ACF
ACF
= 150 mV
TELEFUNKEN Semiconductors
Figure 3.
RTG ( k
100
W )
D
VT
D
Rev. A1, 20-May-96
V
ACF
ACF
Transmit
rms
Receive
Receive
= 8 mV
Mode
Mode
Idle
voltage at its
(707 mV
1000
pp
).